Increased gamma-aminobutyric acid (GABA), homocarnosine and beta-alanine in cerebrospinal fluid of patients treated with gamma-vinyl GABA (4-amino-hex-5-enoic acid)

Metabolomic analyses of vigabatrin (VGB)-treated mice: GABA-transaminase inhibition significantly alters amino acid profiles in murine neural and non-neural tissues

The anticonvulsant vigabatrin (VGB; Sabril^R) irreversibly inhibits GABA transaminase to increase neural GABA, yet its mechanism of retinal toxicity remains unclear. VGB is suggested to alter several amino acids, including homocarnosine, β-alanine, ornithine, glycine, taurine, and 2-aminoadipic acid (AADA), the latter a homologue of glutamic acid. Here, we evaluate the effect of VGB on amino acid concentrations in mice, employing a continuous VGB infusion (subcutaneously implanted osmotic minipumps), dose-escalation paradigm (35-140 mg/kg/d, 12 days), and amino acid quantitation in eye, visual and prefrontal cortex, total brain, liver and plasma. We hypothesized that continuous VGB dosing would reveal numerous hitherto undescribed amino acid disturbances. Consistent amino acid elevations across tissues included GABA, β-alanine, carnosine, ornithine and AADA, as well as neuroactive aspartic and glutamic acids, serine and glycine. Maximal increase of AADA in eye occurred at 35 mg/kg/d (41 ± 2 nmol/g (n = 21, vehicle) to 60 ± 8.5 (n = 8)), and at 70 mg/kg/d for brain (97 ± 6 (n = 21) to 145 ± 6 (n = 6)), visual cortex (128 ± 6 to 215 ± 19) and prefrontal cortex (124 ± 11 to 200 ± 13; mean ± SEM; p < 0.05), the first demonstration of tissue AADA accumulation with VGB in mammal. VGB effects on basic amino acids, including guanidino-species, suggested the capacity of VGB to alter urea cycle function and nitrogen disposal. The known toxicity of AADA in retinal glial cells highlights new avenues for assessing VGB retinal toxicity and other off-target effects.

Disorders of GABA metabolism: SSADH and GABA-transaminase deficiencies

Clinical disorders known to affect inherited gamma-amino butyric acid (GABA) metabolism are autosomal recessively inherited succinic semialdehyde dehydrogenase and GABA-transaminase deficiency. The clinical presentation of succinic semialdehyde dehydrogenase deficiency includes intellectual disability, ataxia, obsessive-compulsive disorder and epilepsy with a nonprogressive course in typical cases, although a progressive form in early childhood as well as deterioration in adulthood with worsening epilepsy are reported. GABA-transaminase deficiency is associated with a severe neonatal-infantile epileptic encephalopathy.

Changes in the ERG d-wave with vigabatrin treatment in a pediatric cohort

PURPOSE

Vigabatrin (VGB), a treatment for the childhood epilepsy, infantile spasms (IS), is implicated in visual field constriction. Electroretinograms (ERGs) are used as a substitute for visual field testing in infants. We use the VGB-associated ERG reduction (VAER), defined as reduction in age-corrected light adapted 30 Hz flicker amplitude from a pre-treatment measurement in the absence of other retinal defects, as an indicator of retinal toxicity resulting from VGB use. The d-wave ERG response is predominantly the result of OFF-bipolar cell depolarization response to light offset. The purpose of this study is to evaluate the ERG d-wave response as a marker for VAER toxicity in an infant population.

METHODS

One hundred children with IS treated with VGB (median age at baseline: 7.6 months; range 1.7-38.4) were tested for the cone-OFF response elicited to a 250 cd s m(2) flash with 200 ms duration (long flash ERG). Diagnosis of VAER requires baseline testing of the flicker ERG and at least one follow up ERG; Fifty-one patients fulfilled this criteria. Fifty-eight children received the long flash ERG at baseline. Thirteen retinally normal controls with a median age of 32 months (5.7-65) were also tested. Amplitude and implicit time of the d-wave response were measured manually.

RESULTS

Longer duration of treatment was associated with reduced d-wave amplitude (ANOVA p < 0.05) in patients taking VGB. Nine patients demonstrated VAER during the course of the study. D-wave amplitude was reduced in the IS group with VAER compared to those without VAER (p < 0.05).

CONCLUSIONS

Vigabatrin associated retinal defects may be reflected in reduction of the cone d-wave amplitude.

Vigabatrin and mental retardation in tuberous sclerosis: infantile spasms versus focal seizures

Tuberous sclerosis complex is a genetic disorder resulting in epilepsy and mental retardation. Vigabatrin has shown efficacy in the treatment of infantile spasms caused by tuberous sclerosis complex, but its effects on focal seizures caused by tuberous sclerosis complex have not been determined. We compared the efficacy of vigabatrin in patients with tuberous sclerosis complex-induced focal seizures and infantile spasms and assessed the mental outcomes in both groups. We retrospectively evaluated 31 children with tuberous sclerosis complex and epilepsy, who were treated with vigabatrin in a single tertiary center in Seoul, Korea. Vigabatrin treatment resulted in spasms cessation in 16 of 18 (88.9%) patients with infantile spasms, whereas 6 of 13 (46.2%) patients with focal seizures became seizure free. Initial response to vigabatrin had no effect on intellectual disability. Vigabatrin was highly effective in eliminating infantile spasms caused by tuberous sclerosis complex but was less effective in patients with focal seizures.

Non-vision adverse events with vigabatrin therapy

Vigabatrin is an effective antiepileptic drug (AED) for the treatment of refractory complex partial seizures (rCPS) and infantile spasms (IS). In clinical trials, vigabatrin was generally well-tolerated with an adverse event profile similar to that of other AEDs. The most common treatment-related adverse events were central nervous system effects, including drowsiness, dizziness, headache, and fatigue, with adjunctive vigabatrin in adults with rCPS, and sedation, somnolence, and irritability with vigabatrin monotherapy in infants with IS. Vigabatrin had little effect on cognitive function, mood, or behavior in a battery of neuropsychologic tests for rCPS. In placebo-controlled clinical trials, the incidence of depression and psychosis, but not other psychiatric adverse events, was greater with vigabatrin than placebo. Intramyelinic edema (IME) was initially identified in rats and dogs and led to a temporary suspension of clinical trials in the United States. IME was subsequently correlated with delays in evoked potential (EP) and increased T(2) -weighted signals on magnetic resonance imaging (MRI). Clinical trials of vigabatrin were allowed to resume after IME was not detected by neuropathologic assessments of autopsy and neurosurgical specimens or by serial EP or MRI assessments in older children and adults receiving vigabatrin. Subsequently, MRI abnormalities characterized by increased T(2) intensity and restricted diffusion were identified in infants treated with vigabatrin for IS. These abnormalities generally resolved with discontinuation of vigabatrin and, in some cases, during continued therapy. The benefit of improved seizure control must be balanced against the potential risks associated with vigabatrin, including abnormal MRI changes and other vigabatrin-related safety issues.

Novel mechanisms of action of three antiepileptic drugs, vigabatrin, tiagabine, and topiramate

Epilepsy, a functional disturbance of the CNS and induced by abnormal electrical discharges, manifests by recurrent seizures. Although new antiepileptic drugs have been developed during recent years, still more than one third of patients with epilepsy are refractory to treatment. Therefore, the search for new mechanisms that can regulate cellular excitability are of utmost importance. Three currently available drugs are of special interest because they have novel mechanisms of action and are especially effective for partial onset seizures. Vigabatrin is a selective and irreversible GABA-transaminase inhibitor that greatly increases whole-brain levels of GABA. Tiagabine is a potent inhibitor of GABA uptake into neurons and glial cells. Topiramate is considered to produce its antiepileptic effect through several mechanisms, including modification of Na(+)-and/or Ca(2+)-dependent action potentials, enhancement of GABA-mediated Cl- fluxes into neurons, and inhibition of kainate-mediated conductance at glutamate receptors of the AMPA/kainate type. This review will discuss these mechanisms of action at the cellular and molecular levels.

Vigabatrin protects against hippocampal damage but is not antiepileptogenic in the lithium-pilocarpine model of temporal lobe epilepsy

In temporal lobe epilepsy (TLE), the nature of the structures involved in the development of the epileptogenic circuit is still not clearly identified. In the lithium-pilocarpine model, neuronal damage occurs both in the structures belonging to the circuit of initiation and maintenance of the seizures (forebrain limbic system) as well as in the propagation areas (cortex and thalamus) and in the circuit of remote control of seizures (substantia nigra pars reticulata). In order to determine whether protection of some brain areas could prevent the epileptogenesis induced by status epilepticus (SE) and to identify the cerebral structures involved in the genesis of TLE, we studied the effects of the chronic exposure to Vigabatrin (gamma-vinyl-GABA, GVG) on neuronal damage and epileptogenesis induced by lithium-pilocarpine SE. The animals were subjected to SE and GVG treatment (250 mg/kg) was initiated at 10 min after pilocarpine injection and maintained daily for 45 days. These pilo-GVG rats were compared with rats subjected to SE followed by a daily saline treatment (pilo-saline) and to control rats not subjected to SE (saline-saline). GVG treatment induced a marked, almost total neuroprotection in CA3, an efficient protection in CA1 and a moderate one in the hilus of the dentate gyrus while damage in the entorhinal cortex was slightly worsened by the treatment. All pilo-GVG and pilo-saline rats became epileptic after the same latency. Glutamic acid decarboxylase (GAD67) immunoreactivity was restored in pilo-GVG rats compared with pilo-saline rats in all areas of the hippocampus, while it was increased over control levels in the optical layer of the superior colliculus and the substantia nigra pars reticulata. Thus, the present data indicate that neuroprotection of principal cells in the Ammon's horn of the hippocampus is not sufficient to prevent epileptogenesis, suggesting that the hilus and extra-hippocampal structures, that were not protected in this study, may play a role in the genesis of spontaneous recurrent seizures in this model. Furthermore, the study performed in non-epileptic rats indicates that chronic treatment with a GABAmimetic drug upregulates the expression of the protein GAD67 in specific areas of the brain, independently from the seizures.

Safety and efficacy of vigabatrin and carbamazepine in newly diagnosed epilepsy: a multicentre randomised double-blind study. Vigabatrin European Monotherapy Study Group

BACKGROUND

Vigabatrin is a newly licensed drug for use in patients with epilepsy. We investigated whether this drug was comparable to standard first-line monotherapy in efficacy and incidence of adverse events.

METHODS

We enrolled 459 patients with newly diagnosed, previously untreated partial epileptic seizures from 44 European centres and randomly assigned them carbamazepine 600 mg daily (n=230) or vigabatrin 2 g daily (n=229). After initial maintenance doses were reached, doses were adjusted downwards (in the case of adverse events) or upwards (in the case of seizures) by the clinician. The primary outcome was time to withdrawal because of lack of efficacy or adverse events. Secondary outcomes included efficacy (time to 6-month remission of seizures, time to first seizure after initial dose stabilisation), and adverse events (incidence and severity). Analysis was by intention to treat.

FINDINGS

Time to withdrawal for lack of efficacy or adverse events did not differ between groups (p=0.318). Vigabatrin was better tolerated than carbamazepine with fewer withdrawals, but was more frequently associated with psychiatric symptoms (58 [25%] vs 34 [15%]) and weight gain (25 [11%] vs 12 [5%]). Carbamazepine was associated with rash (22 [10%] vs seven [3%]). All efficacy outcomes favoured carbamazepine and failed to show equivalence between the two drugs. No significant difference was found for time to achieve 6 months of remission from seizures (p=0.058), but the most powerful outcome, time to first seizure after the first 6 weeks from randomisation, showed carbamazepine to be significantly more effective than vigabatrin (p=0.0001).

INTERPRETATION

Vigabatrin seems less effective but better tolerated than carbamazepine, which is the first-choice drug for the treatment of partial epilepsies. Vigabatrin cannot therefore be recommended as a first-line drug for monotherapy in this group of patients.

Aminoaciduria resulting from vigabatrin administration in children with epilepsy

Vigabatrin (gamma-vinyl gamma aminobutyric acid), a recently developed antiepileptic drug, has been extensively evaluated in the treatment of drug-resistant epilepsy. Several case reports demonstrated that vigabatrin affects urinary excretion of several amino and organic acids. Fourteen children were investigated for the presence of abnormal urinary amino acids before and after treatment with vigabatrin. All demonstrated increased urinary excretion of amino acids, particularly beta-alanine, gamma-aminobutyric acid, and beta-aminoisobutyric acid while on vigabatrin, which were not detected when off medication. These results emphasize the importance of obtaining urine for metabolic evaluation before the administration of vigabatrin.

Acute effects of vigabatrin on brain GABA and homocarnosine in patients with complex partial seizures

PURPOSE

The acute, subacute, and chronic effects of vigabatrin (VGB) were studied in patients with refractory complex partial seizures. VGB increases human brain gamma-aminobutyric acid (GABA) and the related metabolites, homocarnosine and 2-pyrrolidinone.

METHODS

In vivo measurements of GABA and homocarnosine were made of a 14-cc volume in the occipital cortex by using 1H spectroscopy with a 2.1-Tesla magnetic resonance spectrometer and an 8-cm surface coil. Six patients (three women) were studied serially during the initiation and maintenance of VGB as adjunct therapy.

RESULTS

The first, 3 g dose of VGB increased brain GABA by 2.0 micromol/g within 81 min of oral administration. After 2 h, median edited GABA remained essentially the same for 2 days. The response to the second, 3-g dose of VGB given at 48 h was considerably less than that to the first dose, with a median increase of 0.5 micromol/g within 72 min. After 2-3 months, rechallenging patients taking 1.5-g VGB twice daily with 6 g increased GABA by 0.4 micromol/g within 87 min. Homocarnosine increased more gradually than GABA to above-normal levels after a week of VGB therapy.

CONCLUSIONS

VGB promptly elevates brain GABA and presumably offers partial protection against further seizures within hours of the first oral dose. Once-a-day dosing is sufficient to increase GABA. Patients may be expected to experience the effects of increased homocarnosine within 1 week.

Changes in plasma GABA concentration during vigabatrin treatment of epilepsy: a prospective study

The aim of the present prospective study was to evaluate changes in plasma GABA concentration in relation to clinical response during vigabatrin treatment of epilepsy. We studied 29 patients with uncontrolled partial-onset seizures during open add-on vigabatrin treatment and measured plasma GABA and vigabatrin concentrations by a sensitive HPLC method. Following short-term treatment 17 out of 28 patients had a seizure reduction of > 50% (responders). After long-term treatment 16 out of 22 patients were responders. There was no difference between responders and nonresponders regarding pretreatment seizure frequency, treatment duration, vigabatrin dose, or plasma vigabatrin concentration. Responders had a significant (p < 0.001) increase in mean plasma GABA both after short-term (from 0.380 to 0.530 nmol/ml; mean increase: 48%) and after long-term (from 0.392 to 0.618 nmol/ml; mean increase: 71%) vigabatrin treatment, whilst nonresponders had no significant changes in GABA levels. However, since plasma GABA increased in a subgroup of nonresponders, mean plasma GABA levels did not differ between responders and nonresponders. Although plasma GABA increased significantly in the responder but not in the nonresponder group during vigabatrin treatment of patients with epilepsy, it does not seem to be a reliable marker of individual clinical response to vigabatrin treatment.

Effects of increasing doses of vigabatrin on platelet gamma-aminobutyric acid-transaminase and brain gamma-aminobutyric acid in rats

We report the relationship of GABA-transaminase inhibition in platelets and brain with the increase in brain gamma-aminobutyric acid (GABA), as percents of the control, at 24 h after single and after 3 and 8 days of treatment with increasing doses (1, 3, 10, 30, 100 and 300 mg kg(-1) day(-1) of vigabatrin in rats. The inhibition of GABA-transaminase in platelets correlated at least as well as that in brain with the increase in brain GABA after 3 days (r = - 0.87 vs. r = -0.78), and 8 days of treatment (r = -0.77 vs. r = -0.74), and when the data of single and multiple doses were pooled (r = -0.77 vs. r = -0.75). The correlation between platelet GABA-transaminase and brain GABA fitted to a power curve, the increase in brain GABA being significant only when platelet GABA-transaminase was inhibited to less than 50% of the control. Our results suggest that platelet GABA-transaminase could be a peripheral marker of the effect of vigabatrin on brain GABA in rats.

Dose-Response Study of Vigabatrin as add-on therapy in patients with uncontrolled complex partial seizures

PURPOSE

This placebo-controlled, randomized, double-blind, multicenter study examined the efficacy and safety of three daily doses of vigabatrin (VGB; 1, 3, or 6 g) as add-on therapy in 174 patients with previously uncontrolled complex partial seizures with or without secondary generalization.

METHODS

A 12-week pretreatment assessment period was followed by drug therapy with a 6-week titration period and a 12-week maintenance phase.

RESULTS

VGB doses of 3 and 6 g/day reduced median monthly frequency of seizures by 4.3 and 4.5 seizures, respectively, compared with 0.2 seizures for placebo (p = 0.0001). The percentages of patients classified as therapeutic successes (> or =50% reduction in seizure frequency) were 7% for placebo and 24, 51, and 54% for patients taking daily VGB doses of 1, 3, and 6 g, respectively; the comparison with placebo was significant for all treatment groups. The linear trend for dose response was highly significant (p< or =0.0001) for both median monthly seizure frequency and therapeutic success. Vigabatrin was well tolerated, causing no clinically significant changes in laboratory parameters, brain magnetic resonance imaging, evoked potentials, cognitive function, or psychosocial tests. Fatigue, drowsiness, and dizziness were the most common treatment-related adverse events in all treatment groups. Dropouts due to adverse events were higher in the 6-g/day group.

CONCLUSIONS

VGB was significantly more effective than placebo as add-on therapy in reducing seizure frequency. VGB at 3 and 6 g/day produced the best efficacy: however, adverse events may limit the use of the 6-g/day dose in some patients.

Vigabatrin increases human brain homocarnosine and improves seizure control

Homocarnosine, a dipeptide of gamma-aminobutyric acid (GABA) and histidine, is thought to be an inhibitory neuromodulator synthesized in subclasses of GABAergic neurons. Homocarnosine is present in human brain in greater amounts (0.4-1.0 micromol/g) than in other animals. The antiepileptic drug vigabatrin increases human cerebrospinal fluid homocarnosine linearly with daily dose. By using 1H nuclear magnetic resonance spectroscopy, serial occipital lobe GABA and homocarnosine concentrations were measured in 11 patients started on vigabatrin. Daily low-dose (2 g) vigabatrin increased both homocarnosine and GABA. Larger doses of vigabatrin (4 g) further increased homocarnosine but changed GABA levels minimally. Seizure control improved with increasing homocarnosine and GABA concentrations. Patients whose seizure control improved with the addition of vigabatrin had higher mean homocarnosine, but the same mean GABA concentrations, than those whose seizure control did not improve. Increased homocarnosine may contribute to improved seizure control.

Measuring human brain GABA in vivo: effects of GABA-transaminase inhibition with vigabatrin

Gamma-aminobutyric acid (GABA) plays a pivotal role in suppressing the origin and spread of seizure activity. Low occipital lobe GABA was associated with poor seizure control in patients with complex partial seizures. Vigabatrin irreversibly inhibits GABA-transaminase, raising brain and cerebrospinal fluid (CSF) GABA concentrations. The effect of vigabatrin on occipital lobe GABA concentrations was measured by in vivo nuclear magnetic-resonance spectroscopy. Using a single oral dose of vigabatrin, the rate of GABA synthesis in human brain was estimated at 17% of the Krebs cycle rate. As the daily dose of vigabatrin was increased to up to 3 g, the fractional elevation of brain GABA was similar to CSF increase. Doubling the daily dose from 3 to 6 g failed to increase brain GABA further. Increased GABA concentrations appear to reduce GABA synthesis in humans as it does in animals. With traditional antiepileptic drugs, remission of the seizure disorder was associated with normal GABA levels. With vigabatrin, elevated CSF and brain GABA was associated with improved seizure control. Vigabatrin enhances the vesicular and nonvesicular release of GABA. The release of GABA during seizures may be mediated in part by transporter reversal that may serve as an important protective mechanism. During a seizure, this mechanism may be critical in stopping the seizure or preventing its spread.

The influence of established and new antiepileptic drugs on visual perception. II. A controlled study in patients with epilepsy under long-term antiepileptic medication

In this study, we investigated visual performance under chronic antiepileptic drug treatment. Patients were under carbamazepine (CBZ) (n = 18), valproic acid (VPA) (n = 9), CBZ and vigabatrin (VGB) (n = 4), CBZ and gabapentin (GBP) (n = 8), and under CBZ and topiramate (TPR) (n = 6), respectively. Seven untreated patients with epilepsy and 42 healthy volunteers served as controls. The test battery comprised the Lanthony-D15-désaturé colour perception test, increment, postadaptation and transient tritanopia (TT) threshold measurements, visual perception threshold assessments for monochromatic and chromatic gratings and gaussian dots, and critical flicker fusion (CFF) tests. No differences were seen between naive patients and healthy controls. Patients under drug treatment always showed alterations of visual perception. Postadaptation and TT thresholds were altered under each drug regimen after short delays between switching off the adaptation light and switching on the blue test light. Threshold elevations were maximum under the combination of CBZ and TPR and lowest under CBZ and GBP. Consistent impairment of the CFF was seen under combined CBZ and TPR whereas VPA as well as combined CBZ and VGB led to ameliorations the mechanisms of which are discussed. The other tests were less sensitive. In conclusion, alterations of visual function were apparent under chronic antiepileptic drug treatment both with established and new agents. However, it may be difficult to distinguish between effects based on specific modes of action and nonspecific retino- and neurotoxicity.

Comparison of the preclinical anticonvulsant profiles of tiagabine, lamotrigine, gabapentin and vigabatrin

Tiagabine is a novel antiepileptic drug which has clinical efficacy against complex refractory and myoclonic seizures. The anticonvulsant mechanism of action of tiagabine results from its blockade of neuronal and glial GABA-uptake, thereby increasing GABA levels in the synaptic cleft. Here we present a comparison of the preclinical anticonvulsant profile of tiagabine with that of lamotrigine, gabapentin and vigabatrin in the following tests (all antiepileptic drugs were administered i.p.): seizures induced by pentylentetrazol (PTZ), 6,7-dimethoxy-4-ethyl-b-carboline-3-carboxylate (DMCM) and maximal electroshock (MES); sound induced seizures in DBA/2 mice and finally acute amygdala kindled seizures. Tiagabine was the most potent drug in antagonizing tonic convulsions induced by PTZ, DMCM and sound induced seizures in DBA/2 mice with ED50 values of 2, 2 and 1 mumol/kg, respectively, followed by lamotrigine with ED50 values of 9, 43 and 6 mumol/kg, respectively. Gabapentin and vigabatrin had ED50 values in the same tests of 185, 452, 66 mumol/kg and 2322, > 7740, 3883 mumol/kg, respectively. Tiagabine was the only drug capable of blocking PTZ-induced clonic convulsions (ED50 = 5 mumol/kg), an effect seen at low but not high doses of tiagabine. Lamotrigine was the only drug which antagonized tonic convulsions in the MES test (ED50 = 36 mumol/kg). Therapeutic index (TI) of antiepileptic drugs in NMRI- and DBA/2-mice ranked with decreasing TI lamotrigine > gabapentin > vigabatrin > tiagabine. All drugs reduced the generalized seizures in amygdala kindled rats, but tiagabine and gabapentin furthermore attenuated afterdischarge duration of amygdala kindled seizures. However, an ED50 value against amygdala kindled focal seizures was only obtained for tiagabine (36 mumol/kg). The data here presented show that tiagabine, lamotrigine, gabapentin and vigabatrin possess different preclinical anticonvulsant profiles which is of relevance to the clinical anticonvulsant profiles of the drugs.

A double-blind, placebo-controlled crossover study of vigabatrin 2 g/day and 3 g/day in uncontrolled partial seizures

The efficacy and tolerability of vigabatrin as add-on therapy was assessed in patients with uncontrolled partial seizures. Ninety-seven patients entered this seven-centre, double-blind, placebo-crossover study. Vigabatrin (2 g or 3 g) or placebo was administered daily. Vigabatrin was well-tolerated and did not cause clinically significant adverse drug effects when added to established anticonvulsant therapy. No significant differences were observed between dose groups for the overall incidence of adverse events, although drowsiness and visual disturbances (diplopia, ataxia, visual abnormalities) showed a dose-related increase with vigabatrin treatment. The results of this study indicate that vigabatrin, given in a daily dose of either 2 g or 3 g is significantly more effective than placebo in reducing seizure frequency among patients with partial seizures.

Neurochemical and morphological changes associated with human epilepsy

To date a multitude of studies into the morphology and neurochemistry of human epilepsy have been undertaken with variable, and often inconsistent, results. This review summarises these studies on a range of neurotransmitters, neuromodulators, neuropeptides and their receptors. In addition to this, novel changes in cell viability and sprouting have been identified and are discussed. Whether the alterations observed are a result of the seizures or are a contributory factor is unclear. However, it may be that following an initial insult (such as febrile convulsions, status epilepticus or head injury) secondary processes occur both of an anticonvulsant nature in an attempt to compensate for seizure activity, and in a kindling type of fashion, resulting in an increased susceptibility to seizures, leading to future seizures. Many of the alterations documented in this study probably represent one or both of these processes. Clearly no single chemical abnormality or morphological alteration is going to explain the clinically diverse disorder of epilepsy. However, by drawing together the neurochemistry and morphology of epilepsy, we may begin to understand the mechanisms involved in seizure disorders.

Antiepileptic drug mechanisms of action

Established antiepileptic drugs (AEDs) decrease membrane excitability by interacting with neurotransmitter receptors or ion channels. AEDs developed before 1980 appear to act on sodium channels, gamma-aminobutyric acid type A (GABAA) receptors, or calcium channels. Benzodiazepines and barbiturates enhance GABAA receptor-mediated inhibition. Phenytoin (PHT), carbamazepine (CBZ), and possibly valproate (VPA) decrease high-frequency repetitive firing of action potentials by enhancing sodium-channel inactivation. Ethosuximide (ESM) and VPA reduce a low threshold (T-type) calcium-channel current. The mechanisms of action of the new AEDs are not fully established. Gabapentin (GBP) binds to a high-affinity site on neuronal membranes in a restricted regional distribution of the central nervous system. This binding site may be related to a possible active transport process of GBP into neurons; however, this has not been proven, and the mechanism of action of GBP remains uncertain. Lamotrigine (LTG) decreases sustained high-frequency repetitive firing of voltage-dependent sodium action potentials that may result in a preferential decreased release of presynaptic glutamate. The mechanism of action of oxcarbazepine (OCBZ) is not known; however, its similarity in structure and clinical efficacy to CBZ suggests that its mechanism of action may involve inhibition of sustained high-frequency repetitive firing of voltage-dependent sodium action potentials. Vigabatrin (VGB) irreversibly inhibits GABA transaminase, the enzyme that degrades GABA, thereby producing greater available pools of presynaptic GABA for release in central synapses. Increased activity of GABA at postsynaptic receptors may underline the clinical efficacy of VGB.

Development of myoclonus in patients with partial epilepsy during treatment with vigabatrin: an electroencephalographic study

In the context of a study of the effects of gamma-vinyl-GABA (GVG) on seizure occurrence and on EEG abnormalities we present three cases with focal epilepsy in which new clinical and EEG paroxysmal manifestations were observed during GVG therapy. At that time, whereas an amelioration or no change in patients' habitual seizures were observed, myoclonic jerks appeared with related changes in the EEG paroxysmal abnormalities, represented by generalized polyspike and wave complexes. An electroclinical correlation was recorded in one case. These data indicate that, although occurring rarely, it is possible to have epileptic myoclonus during GVG treatment. Mechanisms underlying these manifestations are difficult to explain. Probably a shift in the anti/proconvulsant GABAergic balance towards the latter may compromise the therapeutic effect of GVG.

A comparison of population and standard two-stage pharmacokinetic analyses of vigabatrin data

Vigabatrin (VGB), an irreversible inhibitor of GABA, is being developed as an add-on therapy for uncontrolled complex partial seizure. A single-dose study was conducted in three groups of subjects with normal, mild-to-moderate, and moderate-to-severe renal impairment to examine the effect of renal function on the pharmacokinetics of VGB. Serial blood samples were collected up to 60 h following a single 750 mg oral dose of VGB for the quantitation of drug concentrations. The plasma VGB concentration-time data were analyzed by mixed-effects modeling to estimate population pharmacokinetic parameters and to identify any significant demographic covariates. The parameters of VGB were also calculated by standard two-stage techniques and then compared to the results obtained using the mixed-effects analysis. Population VGB plasma concentration-time profiles were best described by a two-compartment model with zero-order absorption. Creatinine clearance was observed to significantly affect the oral clearance of VGB (p < 0.05), i.e. a linear increasing relationship existed between the two variables. Other demographic factors had no influence on VGB pharmacokinetics. There were agreements in the oral clearance, apparent volume of distribution during elimination, and half-life estimates calculated by both methods. In addition, the conventional technique identified a linear relationship between oral and creatinine clearances. In summary, mixed-effects modeling of serial vigabatrin data validated results determined by the standard two-stage technique.

Newer antiepileptic drugs. Towards an improved risk-benefit ratio

Epilepsy is one of the most common neurological disorders. Even though existing antiepileptic drugs can render 80% of newly diagnosed patients seizure free, a significant number of patients have chronic intractable epilepsy causing disability with considerable socioeconomic implications. There is, therefore, a need for more potent and effective antiepileptic drugs and drugs with fewer adverse effects, particularly CNS effects. Drugs for the treatment of partial seizures are particularly needed. With major advances in our understanding of the basic neuropathology, neuropharmacology and neurophysiology of epilepsy, numerous candidate novel antiepileptic drugs have been developed in recent years. This review comparatively evaluates the pharmacokinetics, efficacy and adverse effects of 12 new antiepileptic drugs namely vigabatrin, lamotrigine, gabapentin, oxcarbazepine, felbamate, tiagabine, eterobarb, zonisamide, remacemide, stiripentol, topiramate and levetiracetam (ucb-L059). Of the 12 drugs, vigabatrin, lamotrigine and gabapentin have recently been marketed in the UK. Five of these new drugs have known mechanisms of action (vigabatrin, lamotrigine, tiagabine, oxcarbazepine and eterobarb), which may provide for a more rational approach to the treatment of epilepsy. Oxcarbazepine, remacemide and eterobarb are prodrugs. Vigabatrin, gabapentin and topiramate are more promising on the basis of their pharmacokinetic characteristics in that they are excreted mainly unchanged in urine and not susceptible to significant pharmacokinetic interactions. In contrast, lamotrigine, felbamate and stiripentol exhibit significant drug interactions. Essentially, all the drugs are effective in partial or secondarily generalised seizures and are effective to varying degrees in other seizure types. Particularly welcome is the possible effectiveness of zonisamide in myoclonus and felbamate in Lennox-Gastaut syndrome. In relation to adverse effects, CNS effects are observed with all drugs, however, gabapentin, remacemide and levetiracetam appear to exhibit least. There is also the possibility of rational duotherapy, using drugs with known mechanisms of action, as an additional therapeutic approach. The efficacy of these 12 antiepileptic drug occurs despite the fact that candidate antiepileptic drugs are evaluated under highly unfavourable conditions, namely as add-on therapy in patients refractory to drug management and with high seizure frequency. Thus, whilst candidate drugs which do become licensed are an advance in that they are effective and/or are associated with less adverse effects than currently available antiepileptic drugs in these patients, it is possible that these drugs may exhibit even more improved risk-benefit ratios when used in normal clinical practice.

Vigabatrin treatment in children

Sixty-nine children, aged from 2 months to 16 years and suffering from different types of drug-resistant epileptic seizures, mostly complex partial and secondary generalised, were recruited in an open, uncontrolled, prospective study of treatment with vigabatrin (gamma-vinyl GABA). Following a 3-month baseline observation period, the initial dose of vigabatrin of 10 mg/kg per day was progressively increased up to a maximum of 140 mg/kg per day, in addition to the conventional concomitant therapy. Sixteen patients showed a > or = 50% reduction in seizure frequency compared with the baseline, with complete control of seizures in nine cases. In 14 other patients, no substantial change in seizure frequency was observed, although an improvement in psychological performance after vigabatrin treatment warranted further continuation of the drug. In 35 patients vigabatrin was discontinued because of lack of efficacy (22 cases) and/or increased seizure frequency (13 cases). The clinical and biological tolerance of vigabatrin was remarkably good.

Effect of long-term vigabatrin therapy on GABA and other amino acid concentrations in the central nervous system--a case study

A 32 year old patient with refractory complex partial seizures was treated with vigabatrin for 3.5 years. Before starting treatment and at 42 months, lumbar punctures were done and the CSF analyzed for amino acids including GABA. Although the patient experienced a 50% seizure reduction, he underwent a left sided temporal lobectomy, and the tissue sample was also analyzed for amino acid content. It was found that vigabatrin caused a three-fold increase in total and free GABA in both the tissue sample and CSF. There were no other significant changes in the other amino acids analyzed. Seizure reduction seen initially was maintained over the long-term observation period. The finding of a specific increase of GABA in brain tissue and CSF of this patient treated with vigabatrin provides additional support to the concept that the primary effect of vigabatrin is as a selective enzyme activated irreversible inhibitor of GABA transaminase.

Pharmacokinetics of vigabatrin following single and multiple oral doses in normal volunteers

The pharmacokinetics of vigabatrin were investigated after single and multiple oral doses in two groups of 24 healthy male volunteers. Vigabatrin was well tolerated by the volunteers; headache was the most frequently reported adverse event. There were no clinically remarkable changes in serum chemistry, urinalysis, or hematology attributable to vigabatrin. For the single-dose study, a stepwise linear contrast method was used to assess dose proportionality. The results showed that vigabatrin exhibited dose linear pharmacokinetics after single oral doses ranging from 0.5 to 4.0 g. Slight changes in the terminal phase half-life and renal clearance were evident in the higher dosage groups. These changes with increasing dose of vigabatrin were relatively minor and not considered to be clinically important. Evaluation of the multiple-dose pharmacokinetics indicated that vigabatrin exhibited dose linearity over the range of 0.5 to 2.0 g administered every 12 hours. The terminal phase half-life and renal clearance of vigabatrin during multiple dosing were consistent with that after single doses. During multiple dosing, steady-state concentrations of vigabatrin were reached on the second day of dosing, and drug accumulation was minimal.

Passage of S(+) and R(-) gamma-vinyl-GABA across the human isolated perfused placenta

1. The maternal to foetal transfers of S(+)- and R(-)-gamma-vinyl-GABA (VGB) across the human isolated perfused placenta were low and comparable with those of acidic alpha-amino acids. 2. The placental uptake of the active S(+)-isomer from the maternal circulation exceeded that of the R(-)-isomer and this was reflected by a corresponding difference in placental tissue concentrations. 3. During perfusion with recirculation of the foetal medium, the two enantiomers were present at a similar concentration and did not concentrate in foetal perfusate, indicating that the excess amount of S(+)-VGB cleared from the maternal circulation was not accessible to the foetal perfusate. Furthermore, stable concentrations of both isomers in the foetal perfusate suggested a lack of placental metabolism. 4. Possible explanations of these findings include the operation of a stereoselective sodium-dependent-GABA placental uptake system on the maternal side, similar to that observed in neuronal tissue, or stereoselective binding to a placental GABA transaminase.

Pharmacology of vigabatrin

Vigabatrin (gamma-vinyl GABA) is a relatively new antiepileptic drug. Vigabatrin increases the concentration of gamma-aminobutyric acid (GABA) in the brain by inhibiting the major GABA metabolizing enzyme, GABA transaminase. Controlled clinical trials have demonstrated an excellent antiepileptic effect of vigabatrin, especially in the treatment of partial epilepsies. Long-term evaluations have shown no signs of tolerance development. Vigabatrin decreases the plasma concentration of phenytoin during concomitant therapy, the only drug with which an interaction seems to occur. In general, vigabatrin is well tolerated. Psychotic reactions occur in 3-6% of patients. Other frequent side effects are sedation and weight increase. Chronic vigabatrin intoxication in animals caused development of intramyelinic oedema, appearing as microvacuoles in brain white matter. No microvacuolation has been observed in humans, even after long-term treatment. Vigabatrin seems a very valuable new antiepileptic drug.

Development of tolerance to the anticonvulsant effect of vigabatrin in amygdala-kindled rats

The anticonvulsant, adverse and biochemical effects of the novel antiepileptic drug vigabatrin (gamma-vinyl GABA), which increases GABA (gamma-aminobutyric acid) levels by inhibition of the GABA degrading enzyme GABA aminotransferase, were examined in amygdala-kindled rats after acute and chronic administration. Vigabatrin proved to be a potent anticonvulsant drug at acute doses (100-200 mg/kg), but during chronic administration, the anticonvulsant activity of the treatment was lost already in the second week of treatment. Tolerance also developed to the adverse effects, i.e. hypothermia, sedation and motor impairment. Determination of vigabatrin in plasma indicated that tolerance was not due to declining drug levels. Furthermore, determination of endogenous amino acids in plasma showed that GABA levels were highly elevated throughout the period of treatment, although the extent of GABA accumulation decreased in the second week. After cessation of chronic treatment with vigabatrin, there was no clear indication of withdrawal symptoms, except a prolonged seizure or afterdischarge duration in experiments with 100 mg/kg per day. The data suggest that chronic treatment with vigabatrin may be associated with a loss of anticonvulsant efficacy, at least when the drug is given as monotherapy.

Relationship between platelet and brain GABA transaminase inhibition by single and multiple doses of vigabatrin in rats

Vigabatrin (gamma-vinyl-GABA, GVG) is an inhibitor of brain GABA transaminase (GABA-T) that also inhibits platelet GABA-T in rats and humans. We have compared the effects of single and multiple doses of GVG on both enzymes in 19 groups of 10 adult male Wistar rats, treated with increasing GVG doses (0-1,600 mg/kg/day) for 1, 8, and 28 days. The platelet GABA-T was more sensitive to the inhibitory effects of GVG than the brain enzyme was especially with low dosages of GVG. After 8 days of treatment, higher GVG plasma levels and a higher inhibition of both enzymes were shown. However, after 28 days, lower GVG plasma levels and similar inhibition of both enzymes compared to the eighth day were found. Correlations between platelet and brain GABA-T for individual rats were statistically significant after 1 day (r = 0.40, p less than 0.01) but not after 8 and 28 days of treatment because of the total inhibition of platelet GABA-T and only partial inhibition of brain GABA-T. We concluded the following: (a) platelet GABA-T is more inhibited than brain GABA-T when low doses of GVG are used and (b) multiple doses reach a higher inhibition of both enzymes than single doses, which could be explained by an increase in GVG concentrations.

Effect of gamma-vinyl gamma-aminobutyric acid on the gamma-aminobutyric acid receptor-coupled chloride ion channel in vesicles from the brain of the rat

The effect of gamma-vinyl GABA on the gamma-aminobutyric acid (GABA) receptor-coupled chloride ion (Cl-) channel was studied using membrane vesicles from cerebral cortex of the rat. gamma-Vinyl GABA, an antiepileptic drug, had no effect on uptake of 36Cl-, without preincubation. However, preincubation of membrane vesicles with gamma-vinyl GABA (100-1000 microM) produced a concentration-dependent decrease in net uptake of 36Cl-. No alteration was observed in basal uptake of 36Cl-. This decrease in net uptake of 36Cl- was not related to desensitization induced by endogenous GABA, which might be increased by gamma-vinyl GABA through selective, irreversible inhibition of GABA-transaminase (GABA-T). Concentration-response curves for GABA showed that preincubation with gamma-vinyl GABA inhibited GABA-stimulated uptake of 36Cl- with no change in ED50. These results indicate that gamma-vinyl GABA may act directly at the GABA/benzodiazepine ionophore complex, as a non-competitive antagonist of GABA.

Effects of vigabatrin (gamma-vinyl GABA) on neurotransmission-related amino acids and on GABA and benzodiazepine receptor binding in rats

The effect of 12-day intraperitoneal i.p. administration of vigabatrin (GVG, gamma-vinyl GABA) to rats on the neurotransmission-related amino acids in various brain regions (cortex, hippocampus, cerebellum, and spinal cord), cisternal fluid (CSF) and blood was studied. Results showed that GVG administration increased the levels of GABA in cortical and subcortical regions of the brain and CSF without affecting GABA and benzodiazepine receptors in the cortex. In addition, a dose-dependent decrease was noted in the concentration of glutamate in the hippocampus and in the concentrations of aspartate and glutamine in the cortex, hippocampus, and cerebellum. The changes in the levels of amino acids in the brain, except for that of GABA, were not reflected in the CSF, however, and the levels of amino acids in discrete brain regions did not show any correlation with those in the serum or in the CSF. The results suggest that GVG administration might suppress development and spread of seizures not only by elevating the level of the inhibitory amino acid GABA, but also by decreasing the levels of excitatory amino acids in the brain.

Administration of vigabatrin (gamma-vinyl-gamma-aminobutyric acid) affects the levels of both inhibitory and excitatory amino acids in rat cerebrospinal fluid

The effect of vigabatrin (gamma-vinyl-gamma-aminobutyric acid), a new anticonvulsant drug, on the transmitter amino acids in rat cisternal CSF was studied. CSF was collected through a permanently implanted polyethylene cannula from freely moving rats at 5, 24, 48, and 96 h after administration of 1,000 mg/kg of vigabatrin. The free gamma-aminobutyric acid (GABA) level was elevated maximally (13.5-fold; p less than 0.01) at 24 h after injection. The homocarnosine (GABA-histidine) level also was increased (123%; p less than 0.01) at 24 h after injection, and its concentration remained at the same level for the next 3 days. Glycine and taurine concentrations had increased [31% (p less than 0.05) and 63% (p less than 0.01), respectively] at 5 h after injection. It is interesting that the levels of glutamate and aspartate increased [330% (p less than 0.05) and 421% (p less than 0.01), respectively] at 96 h after injection, the time when the free GABA level had returned to the baseline concentration and the vigabatrin level was 3% of the maximal concentration. The present study indicates that a single dose of vigabatrin in rats elevates levels of both the inhibitory and excitatory amino acids in CSF. However, the temporal profile of observed changes in relation to vigabatrin injection shows that neither the long-lasting elevation of GABA content nor the increase in glutamate and aspartate levels correlates with the level of vigabatrin in CSF. These findings suggest that the excitatory mechanisms are also augmented following acute administration of vigabatrin, especially when the content of GABA had decreased to the baseline level and the level of vigabatrin was low.

Pharmacokinetics of the individual enantiomers of vigabatrin (gamma-vinyl GABA) in epileptic children

1. The pharmacokinetics of the enantiomers of vigabatrin were investigated after oral administration of a single 50 mg kg-1 dose of the racemate to two groups of six epileptic children (I: 5 months-2 years, II: 4-14 years). 2. The mean (+/- s.d.) values of maximum plasma concentration and area under the plasma concentration-time curve of the R(-) enantiomer were significantly higher than those of S(+) vigabatrin in both groups: R(-) Cmax: 21 +/- 6.6 (I)-41.3 +/- 13.9 (II) vs S(+) Cmax: 13.9 +/- 4.5 (I)-23.8 +/- 12.2 (II) mg l-1; R(-) AUC: 106 +/- 28.5 (I)-147 +/- 34 (II) vs S(+) AUC: 90.9 +/- 27.9 (I)-117 +/- 26 (II) mg l-1 h. In group I, the half-life of the R(-) isomer was significantly shorter than that of the S(+) isomer; in group II, the half-lives were comparable. 3. For the R(-) enantiomer the area under the curve, and the elimination half-life increased linearly with age. 4. During chronic administration (50 mg kg-1 vigabatrin racemate twice a day for 4 days), the morning trough plasma drug concentrations did not increase.

Development of tolerance to the effects of vigabatrin (gamma-vinyl-GABA) on GABA release from rat cerebral cortex, spinal cord and retina

1. The effects of acute and chronic vigabatrin (gamma-vinyl-GABA) (GVG) administration on gamma-aminobutyric acid (GABA) levels and release in rat cortical slices, spinal cord slices and retinas were studied. 2. GVG (250 mgkg-1 i.p.) administered to rats 18 h before death (acute administration) produced an almost 3 fold increase in GABA levels of the cortex and spinal cord and a 6 fold increase in retinal GABA. The levels of glutamate, aspartate, glycine and taurine were unaffected. 3. When GVG (250 mgkg-1 i.p.) was administered daily for 17 days (chronic administration) a similar (almost 3 fold) increase in cortical GABA occurred but the increases in spinal and retinal GABA were reduced by approximately 40%. 4. Acute administration of GVG strikingly increased the potassium-evoked release (KCl 50 mM) of GABA from all three tissues. This enhanced evoked release was reduced by about 50% in tissues taken from rats that had been chronically treated with GVG. 5. Acute administration of GVG reduced GABA-transaminase (GABA-T) activity by approximately 80% in cortex and cord and by 98% in the retina. Following the chronic administration of GVG, there was a trend for GABA-T activities to recover (significant only in cortex). Acute administration of GVG had no effect on glutamic acid decarboxylase (GAD) activity in cortex or spinal cord. However, chronic treatment resulted in significant decreases in GAD activity in both the cortex and cord (35% and 50% reduction respectively). 6. The K-evoked release of glutamate, aspartate, glycine and taurine from cortical slices and the Kevoked release of glycine from spinal slices and retinas were not affected by either acute or chronic GVG treatment. 7. These experiments indicate that GVG treatment increases specifically the K-evoked release of GABA and that tolerance can develop to this enhancing effect of GVG on central GABA release. This tolerance may result from increased feedback inhibition of GAD with a consequent reduction of presynaptic GABA stores.

Clinical relevance of measuring GABA concentrations in cerebrospinal fluid

Determination of GABA concentrations in human cerebrospinal fluid can be used to assess GABAergic activity in the central nervous system. As CSF free GABA concentrations may vary with age, sex, CSF fraction, and collection and storage conditions, careful attention to these factors are necessary to allow interpretation of results. Longitudinal studies to investigate the influence of pharmacological agents on CSF GABA have proven especially useful to define clinical biochemical activity and have been utilized to attribute the anti-epileptic action of vigabatrin, a selective inhibitor of GABA-transaminase, to its effects on brain GABA metabolism.

Chronic toxicity studies with vigabatrin, a GABA-transaminase inhibitor

The GABA-transaminase inhibitor, vigabatrin, has been shown to have a rather low degree of acute toxicity in several animal species. Oral administration of the drug at 1,000 mg/kg/day for 2-4 weeks caused decreased food consumption and weight loss with resultant prostration and death in both rats and dogs. Dosages of 200 mg/kg/day were tolerated for a year without clinical signs in dogs, although rats suffered reduced weight gains and convulsions after 3-4 months when given the drug in the diet. The convulsions continued to occur frequently throughout the one-yr study, but abated 3-4 months after cessation of treatment. The only consistent histopathologic evidence of toxicity in rats and dogs has been the finding of intramyelinic edema (microvacuolation) in the brain, most notably in certain areas of white matter (cerebellum, reticular formation and optic tract in rats and columns of fornix and optic tract in dogs). No lesions were found in the spinal cord or peripheral nervous system. It took several weeks for the microvacuolation to develop, even at high dosages, but it did not continue to progress thereafter, even though a slight effect was noted at dosages as low as 30-50 mg/kg/day after one yr of treatment. The intramyelinic edemia disappeared within a few weeks after treatment was withdrawn. No residual effects were observed in dogs, whereas rats exhibited swollen axons and microscopic mineralized bodies in the cerebellum. Monkeys exhibited no adverse clinical effects except for occasional loose stools at 300 mg/kg/day. After 16 months of oral treatment at 300 mg/kg/day any suggestion of intramyelinic edema was considered to be equivocal, and there was no evidence of any effect in the 50 or 100 mg/kg/day monkeys after 6 yr of treatment. Higher doses caused chronic diarrhea, thus limiting the dosage in this species. Vigabatrin was shown to be well absorbed in rat, dog and man, whereas dose-limited absorption occurred in the monkey. Metabolism is practically nil in all 4 species and the primary elimination pathway is by glomerular filtration. Because vigabatrin is an irreversible inhibitor of GABA-transaminase and the enzyme has a slow turnover rate, plasma levels of the drug are not indicative of its pharmacologic activity. For this reason cerebrospinal fluid levels of GABA and vigabatrin were evaluated, with considerable species differences being noted. The significance of these differences in relation to the differences in toxic response is discussed.

Effect of gamma aminobutyric acid on the carbon dioxide rebreathing response of normal subjects: a study using vigabatrin

Animal studies suggest that gamma aminobutyric acid (GABA) may be an important neurotransmitter in the control of respiration. Vigabatrin, a new drug for the treatment of epilepsy, is thought to exert its effect by increasing GABA concentrations in the brain. To assess the effect of increased GABA concentrations in the brain on human respiration we measured the ventilatory response to carbon dioxide in seven normal subjects after they had taken vigabatrin or placebo for three days in a double blind crossover study. There was no change in either the slope or the intercept of the curve of the ventilatory response to carbon dioxide after vigabatrin by comparison with placebo. This study suggests that GABA does not have an important role in the control of respiration in normal individuals.

Review: Normal and abnormal central nervous system GABA metabolism in childhood

The metabolism and function of central nervous system GABA is briefly reviewed. Hereditary disorders of the GABA metabolism presenting in childhood are discussed with particular emphasis on the recently identified succinic semialdehyde dehydrogenase deficiency and GABA-transaminase deficiency, and on diseases associated with low CSF GABA which await further unravelling. Low CSF GABA concentrations are not always associated with convulsions. A separate section is devoted to the CSF as a tool in the diagnosis of these disorders. Finally, we present a few diagnostic and therapeutic guidelines.

Vigabatrin in the treatment of childhood epilepsies: a single-blind placebo-controlled study

Sixty-one pediatric patients (12-229 months of age) with refractory epilepsy were treated with vigabatrin [gamma-vinyl GABA (GVG)] in a 16-week, single-blind, add-on, placebo-controlled trial. Twenty-three patients (38%) showed a reduction of more than 50% in seizure frequency; 12 patients (20%) experienced a seizure increase; and the remaining 26 did not show significant differences between placebo and GVG treatment. Among the 216 patients who entered the long-term phase after having experienced more than 50% decrease in seizure frequency, 14 continued with the same degree of improvement after 2-11 months of follow-up (mean 7.7). GVG was particularly efficient in cryptogenic partial epilepsy. Conversely, nonprogressive myoclonic epilepsy tended to be aggravated. Agitation was the most commonly observed side effect, mainly at onset of therapy in mentally retarded patients, but was easily reversed by dose reduction. GVG is a promising drug in the treatment of refractory epilepsies of childhood.